The aim of this study was to characterize:e the synthesis and metabolism of platelet activating factor (PAF, 1-0 alkyl-2-0 acetyl-sn-3-phosphorylcholine) by colonic mucosa from patients with ulcerative colitis and healthy individuals. Tissue was obtained by endoscopic biopsy and by scraping the mucosa from surgical resections. Tissue was assayed for the various molecular species of PAF and its biologically inactive metabolite lyso-PAF using gas chromatography/mass spectrometry. Mucosa from surgical resections for ulcerative colitis contained CI6:0 PAF (mean=156 ng/g of mucosa), but not CI&0 PAF. PAF was not identified in mucosa from normal surgical resections or in endoscopic biopsies from either patients with ulcerative colitis or normal individuals. Both C16:0 lyso-PAF and C1:0 lyso-PAF were found in mucosa from normal and ulcerative colitis surgical resections and in endoscopic biopsies from ulcerative colitis and normal tissue. Levels of lyso PAF were similar in ulcerative colitis and normal mucosa. Incubation of mucosa from areas of active inflammation in ulcerative colitis with the calcium ionophore A23 187 increased the levels of CI6:0 PAF by 2-3 fold and also increased the levels of CI6:0 lyso-PAF. Addition of 3H-PAF to endoscopic biopsies from either normal individuals or patients with ulcerative colitis resulted in hydrolysis to 3H-lyso-PAF. The data on colonic mucosal levels of PAF are consistent with the results of earlier studies measuring PAF in patients with ulcerative colitis by bioassay. This study examines the synthesis and metabolism of specific molecular species of PAF in ulcerative colitis for the first time.